The invention relates to an imaging method for spectroscopy of nuclear quadrupole resonances (NQR), especially with polycristalline, powdery solid samples whereby an RF field having a pulse duration t.sub.p with a base amplitude B.sub.10 constant over the sample length for magnetic excitation of nuclear quadrupole resonances with resonance frequencies .omega. and of the magnetic moments coupled to the nuclear quadrupole moments is applied to the sample and whereby the NQR signal emitting from the sample is time-dependently detected.
Such a method is known from an article by Matsui et al. in Journal of Magnetic Resonance 88, 186-191 (1990).
Imaging methods for nuclear magnetic resonance spectroscopy have nowadays become a standard technique for the investigation of samples producing liquidlike signals. There is also progress in the development of methods for imaging of solid materials. The principles common to all these techniques is magnetic resonance expressed by the equation EQU .omega.=.gamma.(B.sub.0 +r.multidot.G) [1]
for the resonance or Larmor frequency .omega..multidot..gamma. is the gyromagnetic ratio, B.sub.0 the external field of the magnet, and r.multidot.G the additional encoding field produced by the gradient coils.
The present invention relates to a method which is based on the second type of nuclear spin resonance, namely nuclear quadrupole resonance (NQR). The attempt to produce images with NQR signals using the normal magnetic field-gradient encoding procedure can lead to severe problems. One then has to consider Zeeman splittings in the weak-field limit. For nuclei with half-integer spins I.gtoreq.3/2, for instance, the zero-field NQR line in the axially symmetric case splits into at least four lines. The splitting depends on the magnetic flux density and the angle between the magnetic field and the electric field gradient. For non-vanishing asymmetry parameters the situation is even more complicated.
Although a linear relationship between .omega. and the magnetic field applied via the encoding gradients is still valid, the usual imaging procedures using phase-encoding fail, like eg. 2DFT-imaging procedures or projection/reconstruction procedures. The Zeeman splitting depends on the orientation of the electric field gradient against the magnetic field. The consequence is an inhomogeneous broadening depending on the magnetic field i.e. on the position. The lines of the quadrupole spectrum tend to overlap or their intensities become too weak to be detectable.
Conventional imaging techniques may only be useful with single crystals with a definite orientation. One applies a weak external magnetic field B.sub.0 to the crystal and chooses the crystal orientation so that the lines are well separated from each other. A suitable line can then be used for encoding with gradients of B.sub.0. Corresponding test experiments have been carried out successfully, but the circumstances under which this procedure is applicable are rather special and limited.
In the above cited publication by Matsui et al. a NQR imaging method is described in which by applying a homogenous magnetic field gradient a further broadening of the quadrupole lines proportionally to the local Zeeman field is produced. At zero field position the line width is at the minimum. The known position-dependence of the line width is then used for imaging in NQR measuring. A disadvantage of this method is that for producing a position-dependent Zeeman field an adequate magnet, normally a high-power radio-frequency (RF) magnetic coil, is necessary. Thereby the whole apparatus is doomed to be relatively big and bulky. Moreover, the sample to be examined has to be smaller than the device, since the sample has to be totally inside the Zeeman field. Another disadvantage of the method is that the position-dependence of the line widths in the magnetic field has to be determined by additional measurements. The recording duration of these additional measuring sequences can be compared with the recording duration of the signals for the actual image reconstruction.